In recent years, plant growth using artificial light sources has been the subject of much research. In particular, cultivation methods using illumination from light-emitting diodes (LED), which exhibit excellent monochromaticity, provide favorable energy conservation and long life and which can be easily miniaturized, have been garnering much attention. Based on the results of research to date, one emission wavelength band that has been confirmed as a suitable light source for plant growth (photosynthesis) is red light having a wavelength within the region from 600 to 700 nm.
In particular, light having a wavelength within the region from 660 to 670 nm is a suitable light source for photosynthesis. Conventional red light-emitting diodes of this wavelength range, having light-emitting layers composed of AlGaAs and InGaNP and the like, have been investigated, however, those has not yet been able to achieve a high output (for example, Patent Documents 1 to 3).
On the other hand, compound semiconductor LEDs having a light-emitting layer composed of an aluminum-gallium-indium phosphide (composition formula: (AlXG1-X)YIn1-YP, 0≦X≦1 and 0<Y≦1) are also known.
Among these LEDs, a light-emitting layer having the composition Ga0.5In0.5P exhibits the longest wavelength, and the peak wavelength obtained from this light-emitting layer is near 650 nm. Accordingly, in the region of the longer wavelength than 655 nm, it is difficult to achieve practical application and a high brightness.
In general, a light-emitting section having a light-emitting layer composed of (AlXGa1-X)YIn1-YP (wherein 0≦X≦1 and 0<Y≦1) is formed on a single crystal substrate gallium arsenide (GaAs) which is optically opaque for light which is emitted from the light-emitting layer and which does not have enough mechanical strength.
Thus, in order to obtain a visible LED with higher luminance, studies for further improving the mechanical strength of the device have been carried out.
It has been disclosed (for example, see Patent Document 4) that after removing the opaque substrate material such as GaAs, a support layer made of a transparent material having increased excellent mechanical strength and being transparent with respect to the emission wavelength was bonded, which is so-called bonding LED technology.
On the other hand, although investigations have been conducted into stained light-emitting layers for laser devices having a different light emission mechanism, there are currently no practical applications of strained light-emitting layers in the field of light-emitting diodes (for example, see Patent Document 5)
Furthermore, investigations are also being pursued into light-emitting sections of light-emitting diodes, which utilize a quantum well structure. However, because the quantum effect obtained by utilizing a quantum well structure shortens the emission wavelength, this effect has been unable to be applied to techniques requiring wavelength lengthening (for example, see Patent Document 6).    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. Hei 9-37648    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2002-27831    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2004-221042    Patent Document 4: Japanese Patent (Granted) Publication No. 3230638    Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2000-151024    Patent Document 6: Japanese Patent (Granted) Publication No. 3373561